The invention relates to a 3D shape-measuring apparatus using biaxial anamorphic magnification that, more particularly, uses telecentric property to uniformize resolution ranging from near distance to far distance.
An image-grabbing device for 3D shape-measuring apparatus conventionally comprises a camera lens with fixed focal length associated with a charge coupled device (CCD) camera having fixed resolutions and a fixed size, the CCD camera having fixed viewing angle along the vertical direction and the horizontal direction. Thus, if the object distance is increasingly decreased, the observable field is increasingly reduced while the resolution is increased. On the contrary, if the object distance is increased, the observable field is increased while the resolution is decreased. When the optical axis of CCD camera is oblique with the light projection direction, the resolution along the light projection direction is adversely decreased. This resolution is all the more decreased as the light source distance is increased.
To increase the resolution along the light projection direction, the U.S. Pat. No. 6,046,812 uses an anisotropic magnification cylindrical lens to magnify the image along the light projection direction, which thereby increases the resolution there along. Meanwhile, the image magnification along the perpendicular direction is not changed. To achieve the same results as disclosed above, an assembly of prisms may be also used to obtain anisotropic magnification, as disclosed in the U.S. Pat. No. 4,872,747.
Therefore, the conventionally known methods can increase the resolution along the light projection direction. However, those methods fail to remedy a decrease of the resolution along the light projection direction as the light source distance is increased. This resolution nonuniformity occurs when a near distance resolution is sufficient while a far distance resolution is not sufficient. In the conventional technology, the parameters of the CCD camera further usually determine the resolution along a direction perpendicular to the light projection direction. Therefore, the choice of focal length and object distance has already decided the resolution along the perpendicular direction. If one desires to change the resolution along the perpendicular direction, either the focal length or object distance only can be changed. However, conventional 3D shape-measuring apparatus have their preferable object distance of operation, and the adjustment range of the object distance is therefore relatively limited. As a result, changing the focal length is thus usually implemented. Regardless a change of either the object distance or focal length, the resolution along the light projection direction would accordingly change, therefore, the parameters of an assembly of anisotropic or anamorphic lenses should be modified. Generally, the focal length of an objective lens directly determines the resolution in either direction. For cost considerations, most objective lenses are standard with fixed focal length such as CCTV lenses. Therefore from a design consideration, simultaneously fulfilling the requirements of biaxial resolutions is a difficult task.
FIG. 1 is a schematic view that illustrates the resolution of a conventional method without anamorphic magnification. In FIG. 1, a light projection plane 11 is projected onto a surface of an object 20 to be sensed, meanwhile the configuration of the light 7 reflected from the object 20 toward the CCD camera to be grabbed via equally spaced CCD pixels thereof is also shown. As shown in FIG. 1, without anamorphic magnification, the CCD camera has a substantially large angle of view of the light projection direction. However, a far distance resolution dramatically drops. Moreover, because the depth of field of conventional objective lenses is substantially limited, the measuring precision in the area of far distance is therefore substantially affected.
FIG. 2 is a schematic view that illustrates the use of a cylindrical lens for a resolution improvement as disclosed in the U.S. Pat. No. 6,046,812. In FIG. 2, a light projection plane 11 is projected onto a surface of an object 20 to be sensed, meanwhile the configuration of the light 7 reflected from the object 20 toward the CCD camera to be grabbed via equally spaced CCD pixels thereof is also shown. As shown in FIG. 2, the use of a cylindrical lens reduces the angle of view of the light projection direction, which increases its resolution. However, far distance resolution is still adversely lower than near distance resolution.
It is therefore a principal object of the invention to provide a 3D shape-measuring apparatus using biaxial anamorphic magnification that can anamorphically magnify an image whatever perpendicular to or parallel with a light projection direction. As a result, uniform resolutions along parallel and perpendicular directions with respect to the light projection direction are obtained for 3D measurement while the resolutions further do not decrease as the distance of the light source increases.
To accomplish the above and other objectives, the invention provides a 3D shape-measuring apparatus using biaxial anamorphic magnification that comprises the following elements. A light source projects a light plane or a light ray on a surface of an object to be sensed. The projected light intersects the surface of the object into a light intersection that can be a curve or a point that is reflected toward a curved reflecting mirror (an assembly of telecentric cylindrical lenses is also suitable). After reflecting on the curved reflecting mirror, the reflected light travels through an assembly of first cylindrical lenses into an electrical image-grabbing device (CCD camera) to form an image onto an image sensor, such as a charge coupled device, therein. Thereby, the coordinates of the points hit by the light from the light source, which can be a laser, are calculated. In the invention, either the curved reflecting mirror or the assembly of telecentric cylindrical lenses can adjust the magnification of an image along the light projection direction. The curved reflecting mirror can be a concave mirror with a continuous curvature that reflects the light reflected from the object. The assembly of telecentric cylindrical lenses comprises an assembly of second cylindrical lenses and a planar reflecting mirror that reduce the angle of view of the light projection direction. The focal location of the assembly of telecentric cylindrical lenses further approaches the principal plane of objective lenses. The assembly of first cylindrical lenses adjusts an image magnification along a direction perpendicular to the light projection. Finally, the image is formed on the image sensor (such as a charge coupled device) of the electrical image-grabbing device.
With the invention as described above, under an optical design of anamorphic magnification, a certain level of quality of image formation can be maintained while increasing the resolutions. The observable range of the CCD camera thus can be efficiently changed into a measurable field.